Wouldn't it make more sense for this person to be Mosa, who already has a relationship with Loki (though it could turn into a betrayal later on), who seems to know Elbaf (and thus could've gotten the DF), and who called to say he wouldn't be in touch for a while for an unspecified reason just as we learned that someone is attacking MJ?
True, i forgot about Mosa while writing that,
I totally agree with you that Mosa seems to be a Villain, i even mentioned it recently after seeing Loki's Childhood,
It seems that Oda wants to show how Everyone is against Loki, the entire World doesn't want him,
And then He creates Mosa as Loki's Only Proof that He still seeks Love/Kindness/Approval,
If Oda just wants to portray Loki as Someone who's gonna be a Hero despite how He was treated, then He doesn't need Mosa,
But Existence of such Relationship suggest that Mosa might be Loki's Last Straw, He/She might be a Villain,
However, i'm not sure if Mosa is the one who took that DF, i feel like Mosa is actually the Host of Mu/Imu.
All of Imu Scenes so far show the Hakuba-Equivalent Side of him speaking, and He is now inside Gunko, but We have yet to see the Cavendish-Equivalent Side of Imu speaking,
Mosa imo is Imu's Host that We keep seeing living in Flower Room,
As i mentioned many times, either Host or Guest takes control of Body, but Double-Layered Eyes means They are both Awake/Watching,
So even though it's Imu most of the time, his Host is still witnessing everything & having her own Thoughts/Emotions,
So for example, when Mosa was talking about some Incident, She was probably scared from Revos Invasion the other day, and recently She called Loki because as you said, Mary Geoise is being Attacked, and it's very convenient that the Call only happened after Imu showed up on Elbaph, rather than right before (As soon as Attack Started or before if Mosa was planning the Attack as you suggested)
I still hold to my opinion that Shamrock is the Main Villain here (Our Spandam) with this DF being his Buster Call Equivalent, while Mosa is Main Damsel In Distress (It's complicated though cuz She/He is attached to Imu, it's like Oda combined Robin/Aokiji's Roles into One Character)
Edit: Also it's worth noting that Mosa might have chosen Loki because of his Strong Ties to Xebec, who as We learned recently visited Imu before & talked about his Goals, something that Mosa wd have witnessed if She/He is indeed Imu's Host,